Position location using broadcast digital television signals comprising pseudonoise sequences

ABSTRACT

A user terminal comprises a receiver adapted to receive, at the user terminal, a broadcast digital television signal transmitted by a television transmitter and comprising a pseudo-noise code; and a controller adapted to generate a pseudorange based on the pseudo-noise code; wherein the location of the user terminal is determined based on the pseudorange and a location of the television transmitter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/409,407, “A Simple Modification to theTerrestrial ATSC Digital TV Standard to Overcome Rapidly ChangingMultipath,” by James J. Spilker, Jr. and Jimmy K. Omura, filed Sep. 9,2002.

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/210,847, “Position Location Using Broadcast DigitalTelevision Signals” by James J. Spilker, Jr. and Matthew Rabinowitz,filed Jul. 31, 2002; U.S. patent application Ser. No. 09/932,010,“Position Location using Terrestrial Digital Video Broadcast TelevisionSignals” by Matthew Rabinowitz and James J. Spilker, Jr., filed Aug. 17,2001; U.S. patent application Ser. No. 10/209,578, “Time-GatedNoncoherent Delay Lock Loop Tracking of Digital Television Signals,” byJames J. Spilker, Jr. and Matthew Rabinowitz, filed Jul. 31, 2002; U.S.patent application Ser. No. 10/159,478, “Position Location using GlobalPositioning Signals Augmented by Broadcast Television Signals,” byMatthew Rabinowitz and James J. Spilker, filed May 31, 2002, and U.S.patent application Ser. No. 10/290,984, “Wireless Position LocationUsing the Japanese ISDB-T Digital TV Signals,” by James J. Spilker, Jr.and Matthew Rabinowitz, filed Nov. 8, 2002.

The foregoing applications are hereby incorporated herein by reference.

BACKGROUND

The present invention relates generally to position determination, andparticularly to position determination using DTV signals.

There have long been methods of two-dimensional latitude/longitudeposition location systems using radio signals. In wide usage have beenterrestrial systems such as Loran C and Omega, and a satellite-basedsystem known as Transit. Another satellite-based system enjoyingincreased popularity is the Global Positioning System (GPS).

Initially devised in 1974, GPS is widely used for position location,navigation, survey, and time transfer. The GPS system is based on aconstellation of 24 on-orbit satellites in sub-synchronous 12 hourorbits. Each satellite carries a precision clock and transmits apseudo-noise signal, which can be precisely tracked to determinepseudo-range. By tracking 4 or more satellites, one can determineprecise position in three dimensions in real time, world-wide. Moredetails are provided in B. W. Parkinson and J. J. Spilker, Jr., GlobalPositioning System-Theory and Applications, Volumes I and II, AIAA,Washington, D.C. 1996.

GPS has revolutionized the technology of navigation and positionlocation. However in some situations, GPS is less effective. Because theGPS signals are transmitted at relatively low power levels (less than100 watts) and over great distances, the received signal strength isrelatively weak (on the order of −160 dBw as received by anomni-directional antenna). Thus the signal is marginally useful or notuseful at all in the presence of blockage or inside a building.

There has even been a proposed system using conventional analog NationalTelevision System Committee (NTSC) television signals to determineposition. This proposal is found in a U.S. patent entitled “LocationDetermination System And Method Using Television Broadcast Signals,”U.S. Pat. No. 5,510,801, issued Apr. 23, 1996. However, the presentanalog TV signal contains horizontal and vertical synchronization pulsesintended for relatively crude synchronization of the TV set sweepcircuitry. Further, in 2006 the Federal Communication Commission (FCC)will consider turning off NTSC transmitters and reassigning thatvaluable spectrum so that it can be auctioned for other purposes deemedmore valuable.

SUMMARY

I will complete this section after your review of this draft.

Advantages that can be seen in implementations of the invention includeone or more of the following.

Advantages that can be seen in implementations of the invention includeone or more of the following. Implementations of the invention may beused to position cellular telephones, wireless PDA's (personal digitalassistant), pagers, cars, OCDMA (orthogonal code-division multipleaccess) transmitters and a host of other devices. Implementations of theinventions make use of a DTV signal which has excellent coverage overthe United States, and the existence of which is mandated by the FederalCommunication Commission. Implementations of the present inventionrequire no changes to the Digital Broadcast Stations.

The DTV signal has a power advantage over GPS of more than 40 dB, andsubstantially superior geometry to that which a satellite system couldprovide, thereby permitting position location even in the presence ofblockage and indoors. The DTV signal has roughly six times the bandwidthof GPS, thereby minimizing the effects of multipath. Due to the highpower and low duty factor of the DTV signal used for ranging, theprocessing requirements are minimal. Implementations of the presentinvention accommodate far cheaper, lower-speed, and lower-power devicesthan a GPS technique would require.

In contrast to satellite systems such as GPS, the range between the DTVtransmitters and the user terminals changes very slowly. Therefore theDTV signal is not significantly affected by Doppler effects. Thispermits the signal to be integrated for a long period of time, resultingin very efficient signal acquisition.

The frequency of the DTV signal is substantially lower that that ofconventional cellular telephone systems, and so has better propagationcharacteristics. For example, the DTV signal experiences greaterdiffraction than cellular signals, and so is less affected by hills andhas a larger horizon. Also, the signal has better propagationscharacteristics through buildings and automobiles.

Unlike the terrestrial Angle-of-Arrival/Time-of-Arrival positioningsystems for cellular telephones, implementations of the presentinvention require no change to the hardware of the cellular basestation, and can achieve positioning accuracies on the order of 1 meter.When used to position cellular phones, the technique is independent ofthe air interface, whether GSM (global system mobile), AMPS (advancedmobile phone service), TDMA (time-division multiple access), CDMA, orthe like. A wide range of UHF (ultra-high frequency) frequencies hasbeen allocated to DTV transmitters. Consequently, there is redundancybuilt into the system that protects against deep fades on particularfrequencies due to absorption, multipath and other attenuating effects.

In general, in one aspect, the invention features a user terminalcomprising a receiver adapted to receive, at the user terminal, abroadcast digital television signal transmitted by a televisiontransmitter and comprising a pseudo-noise code; and a controller adaptedto generate a pseudorange based on the pseudo-noise code; wherein thelocation of the user terminal is determined based on the pseudorange anda location of the television transmitter.

Particular implementations can include one or more of the followingfeatures. The broadcast digital television signal is an AmericanTelevision Standards Committee (ATSC) digital television signal. Thepseudonoise code is a Global Positioning System L5 code. Implementationscomprise a processor adapted to determine the location of the userterminal based on the pseudorange and the location of the identifiedtelelvision transmitter. Implementations comprise a time-gateddelay-lock loop adapted to track the broadcast digital televisionsignal.

In general, in one aspect, the invention features a user terminalcomprising a receiver adapted to receive, at the user terminal, abroadcast digital television signal transmitted by a televisiontransmitter and comprising a pseudonoise code; and a controller adaptedto generate a pseudorange based on the broadcast digital televisionsignal, and to identify the television transmitter based on thepseudonoise code; wherein the location of the user terminal isdetermined based on the pseudorange and a location of the identifiedtelelvision transmitter.

Particular implementations can include one or more of the followingfeatures. The broadcast digital television signal is an AmericanTelevision Standards Committee (ATSC) digital television signal. Thecontroller generates the pseudorange based on a known digital sequencecomprising at least one of the pseudonoise code; a Field SynchronizationSegment within an ATSC data frame, and a Synchronization Segment withina Data Segment within an ATSC data frame. The pseudonoise code is aGlobal Positioning System L5 code. Implementations comprise a processoradapted to determine the location of the user terminal based on thepseudorange and the location of the identified telelvision transmitter.Implementations comprise a time-gated delay-lock loop adapted to trackthe broadcast digital television signal.

In general, in one aspect, the invention features a method, apparatus,and computer-readable media comprising receiving, at a user terminal, abroadcast digital television signal transmitted by a televisiontransmitter and comprising a pseudo-noise code; and generating apseudorange based on the pseudo-noise code; wherein the location of theuser terminal is determined based on the pseudorange and a location ofthe telelvision transmitter.

Particular implementations can include one or more of the followingfeatures. The broadcast digital television signal is an AmericanTelevision Standards Committee (ATSC) digital television signal. Thepseudonoise code is a Global Positioning System L5 code. Implementationscomprise determining the location of the user terminal based on thepseudorange and the location of the identified telelvision transmitter.

In general, in one aspect, the invention features a method, apparatus,and computer-readable media comprising receiving, at a user terminal, abroadcast digital television signal transmitted by a televisiontransmitter and comprising a pseudonoise code; generating a pseudorangebased on the broadcast digital television signal; and identifying thetelevision transmitter based on the pseudonoise code; wherein thelocation of the user terminal is determined based on the pseudorange anda location of the identified telelvision transmitter.

Particular implementations can include one or more of the followingfeatures. The broadcast digital television signal is an AmericanTelevision Standards Committee (ATSC) digital television signal. Thepseudorange is generated based on a known digital sequence comprising atleast one of the pseudonoise code; a Field Synchronization Segmentwithin an ATSC data frame, and a Synchronization Segment within a DataSegment within an ATSC data frame. The pseudonoise code is a GlobalPositioning System L5 code. Implementations comprise determining thelocation of the user terminal based on the pseudorange and the locationof the identified telelvision transmitter.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features will beapparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 depicts an implementation of the present invention including auser terminal that communicates over an air link with a base station.

FIG. 2 illustrates an operation of an implementation of the invention.

FIG. 3 depicts the geometry of a position determination using 3 DTVtransmitters.

FIG. 4 depicts an implementation of a sampler for use in taking samplesof received DTV signals.

FIG. 5 depicts an implementation of a noncoherent correlator for use insearching for the correlation peak of the DTV signal samples produced bythe sampler of FIG. 4.

FIG. 6 illustrates a simple example of a position location calculationfor a user terminal receiving DTV signals from two separate DTVantennas.

FIG. 7 depicts the effects of a single hill on a circle of constantrange for a DTV transmitter that is located at the same altitude as thesurrounding land.

FIG. 8 illustrates the structure of the ATSC frame.

FIG. 9 illustrates the structure of the field synchronization segment ofthe ATSC frame.

FIG. 10 illustrates the structure of the data segment of the ATSC frame.

FIG. 11 shows a plot of the gain function for a filter used in producingan ATSC DTV signal.

FIG. 12 depicts an implementation of a monitor unit.

FIG. 13 illustrates one implementation for tracking in software.

FIG. 14 shows a plot of the output of the non-coherent correlator.

FIG. 15 displays an example spectrum for a 1 millisecond sample of thesignal from a KICU channel 52 DTV broadcast from San Jose.

FIG. 16 shows the computed autocorrelation function for the in-phase andquadrature component of the resulting 6 MHz signal.

FIG. 17 shows the characteristics of the 6 MHz signal.

FIG. 18 depicts the results of a simulation of the operation of thecorrelator of FIG. 5.

FIG. 19 shows the general configuration of the digital TV repeater.

FIG. 20 shows an apparatus for adding the PN synchronization signal tothe ATSC signal according to a first approach.

FIG. 21 shows an apparatus for adding the PN synchronization signal tothe ATSC signal according to a second approach.

The leading digit(s) of each reference numeral used in thisspecification indicates the number of the drawing in which the referencenumeral first appears.

DETAILED DESCRIPTION

Introduction

Digital television (DTV) is growing in popularity. DTV was firstimplemented in the United States in 1998. As of the end of 2000, 167stations were on the air broadcasting the DTV signal. As of Feb. 28,2001, approximately 1200 DTV construction permits had been acted on bythe FCC. According to the FCC's objective, all television transmissionwill soon be digital, and analog signals will be eliminated. Publicbroadcasting stations must be digital by May 1, 2002 in order to retaintheir licenses. Private stations must be digital by May 1, 2003. Over1600 DTV transmitters are expected in the United States.

These new DTV signals permit multiple standard definition TV signals oreven high definition signals to be transmitted in the assigned 6 MHzchannel. These new American Television Standards Committee (ATSC) DTVsignals are completely different from the analog NTSC TV signals, aretransmitted on new 6 MHz frequency channels, and have completely newcapabilities.

The inventors have recognized that the ATSC signal can be used forposition location, and have developed techniques for doing so. Thesetechniques are usable in the vicinity of ATSC DTV transmitters with arange from the transmitter much wider than the typical DTV receptionrange. Because of the high power of the DTV signals, these techniquescan even be used indoors by handheld receivers, and thus provide apossible solution to the position location needs of the Enhanced 911(E911) system.

The techniques disclosed herein are also applicable to DTV signals asdefined by the Digital Video Broadcasting (DVB) standard recentlyadopted by the European Telecommunications Standards Institute (ETSI).For example, the techniques described herein can be used with thescattered pilot carrier signals embedded within the DVB signal. The DVBscattered pilot carrier signals are a set of 868 uniformly-spaced pilotcarrier signals, each of which is frequency hopped in a chirp-likefashion over four sequentially-increasing frequencies. These techniquesare also applicable to DTV signals as defined by the Japanese IntegratedService Digital Broadcasting-Terrestrial (ISDB-T). These techniques arealso applicable to other DTV signals, including those which transmit aknown sequence of data.

In contrast to the digital pseudo-noise codes of GPS, the DTV signalsare received from transmitters only a few miles distant, and thetransmitters broadcast signals at levels up to the megawatt level. Inaddition the DTV antennas have significant antenna gain, on the order of14 dB. Thus there is often sufficient power to permit DTV signalreception inside buildings.

Certain implementations of the present invention use only the DTV signalsynchronization codes as opposed to demodulating and decoding the DTV8-ary Vestigial Sideband Modulation (8VSB) data signal. Consequently,the DTV signal can be correlated for a period roughly a million timeslonger than the period of single data symbol. Thus the ability to tracksignals indoors at substantial range from the DTV tower is greatlyexpanded. Furthermore, through the use of digital signal processing itis possible to implement these new tracking techniques in a singlesemiconductor chip.

Some implementations use a pseudo-noise sequence in the DTV signal forposition location. For example, it has been proposed to add apseudo-noise sequence to the ATSC DTV signal in order to improvetelevision reception in the presence of multipath. These implementationsare not limited to the ATSC signal, but encompass any DTV signalcomprising such a pseudo-noise sequence. These implementations aredescribed in detail below.

Referring to FIG. 1, an example implementation 100 includes a userterminal 102 that communicates over an air link with a base station 104.In one implementation, user terminal 102 is a wireless telephone andbase station 104 is a wireless telephone base station. In oneimplementation, base station 104 is part of a mobile MAN (metropolitanarea network) or WAN (wide area network).

FIG. 1 is used to illustrate various aspects of the invention but theinvention is not limited to this implementation. For example, the phrase“user terminal” is meant to refer to any object capable of implementingthe DTV position location described. Examples of user terminals includePDAs, mobile phones, cars and other vehicles, and any object which couldinclude a chip or software implementing DTV position location. It is notintended to be limited to objects which are “terminals” or which areoperated by “users.”

Position Location Performed by a DTV Location Server

FIG. 2 illustrates an operation of implementation 100. User terminal 102receives DTV signals from a plurality of DTV transmitters 106A and 106Bthrough 106N (step 202).

Various methods can be used to select which DTV channels to use inposition location. In one implementation, a DTV location server 110tells user terminal 102 of the best DTV channels to monitor. In oneimplementation, user terminal 102 exchanges messages with DTV locationserver 110 by way of base station 104. In one implementation userterminal 102 selects DTV channels to monitor based on the identity ofbase station 104 and a stored table correlating base stations and DTVchannels. In another implementation, user terminal 102 can accept alocation input from the user that gives a general indication of thearea, such as the name of the nearest city; and uses this information toselect DTV channels for processing. In one implementation, user terminal102 scans available DTV channels to assemble a fingerprint of thelocation based on power levels of the available DTV channels. Userterminal 102 compares this fingerprint to a stored table that matchesknown fingerprints with known locations to select DTV channels forprocessing.

User terminal 102 determines a pseudo-range between the user terminal102 and each DTV transmitter 106 (step 204). Each pseudo-rangerepresents the time difference (or equivalent distance) between a timeof transmission from a transmitter 108 of a component of the DTVbroadcast signal and a time of reception at the user terminal 102 of thecomponent, as well as a clock offset at the user terminal.

User terminal 102 transmits the pseudo-ranges to DTV location server110. In one implementation, DTV location server 110 is implemented as ageneral-purpose computer executing software designed to perform theoperations described herein. In another implementation, DTV locationserver is implemented as an ASIC (application-specific integratedcircuit). In one implementation, DTV location server 110 is implementedwithin or near base station 104.

The DTV signals are also received by a plurality of monitor units 108Athrough 108N. Each monitor unit can be implemented as a small unitincluding a transceiver and processor, and can be mounted in aconvenient location such as a utility pole, DTV transmitters 106, orbase stations 104. In one implementation, monitor units are implementedon satellites.

Each monitor unit 108 measures, for each of the DTV transmitters 106from which it receives DTV signals, a time offset between the localclock of that DTV transmitter and a reference clock. In oneimplementation the reference clock is derived from GPS signals. The useof a reference clock permits the determination of the time offset foreach DTV transmitter 106 when multiple monitor units 108 are used, sinceeach monitor unit 108 can determine the time offset with respect to thereference clock. Thus, offsets in the local clocks of the monitor units108 do not affect these determinations.

In another implementation, no external time reference is needed.According to this implementation, a single monitor unit receives DTVsignals from all of the same DTV transmitters as does user terminal 102.In effect, the local clock of the single monitor unit functions as thetime reference.

In one implementation, each time offset is modeled as a fixed offset. Inanother implementation each time offset is modeled as a second orderpolynomial fit of the formOffset=a+b(t−T)+c(t−T)²  (1)that can be described by a, b, c, and T. In either implementation, eachmeasured time offset is transmitted periodically to the DTV locationserver using the Internet, a secured modem connection or the like. Inone implementation, the location of each monitor unit 108 is determinedusing GPS receivers.

DTV location server 110 receives information describing the phase center(i.e., the location) of each DTV transmitter 106 from a database 112. Inone implementation, the phase center of each DTV transmitter 106 ismeasured by using monitor units 108 at different locations to measurethe phase center directly. In another implementation, the phase centerof each DTV transmitter 106 is measured by surveying the antenna phasecenter.

In one implementation, DTV location server 110 receives weatherinformation describing the air temperature, atmospheric pressure, andhumidity in the vicinity of user terminal 102 from a weather server 114.The weather information is available from the Internet and other sourcessuch as NOAA. DTV location server 110 determines troposphericpropagation velocity from the weather information using techniques suchas those disclosed in B. Parkinson and J. Spilker, Jr. GlobalPositioning System-Theory and Applications, AIAA, Washington, D.C.,1996, Vol. 1, Chapter 17 Tropospheric Effects on GPS by J. Spilker, Jr.

DTV location server 110 can also receive from base station 104information which identifies a general geographic location of userterminal 102. For example, the information can identify a cell or cellsector within which a cellular telephone is located. This information isused for ambiguity resolution, as described below.

DTV location server 110 determines a position of the user terminal basedon the pseudo-ranges and a location of each of the transmitters (step206). FIG. 3 depicts the geometry of a position determination usingthree DTV transmitters 106. DTV transmitter 106A is located at position(x1, y1). The range between user terminal 102 and DTV transmitter 106Ais r1. DTV 106B transmitter is located at position (x2, y2). The rangebetween user terminal 102 and DTV transmitter 106B is r2. DTVtransmitter 106N is located at position (x3, y3). The range between userterminal 102 and DTV transmitter 106N is r3.

DTV location server 110 may adjust the value of each pseudo-rangeaccording to the tropospheric propagation velocity and the time offsetfor the corresponding DTV transmitter 106. DTV location server 110 usesthe phase center information from database 112 to determine the positionof each DTV transmitter 106.

User terminal 102 makes three or more pseudo-range measurements to solvefor three unknowns, namely the position (x, y) and clock offset T ofuser terminal 102. In other implementations, the techniques disclosedherein are used to determine position in three dimensions such aslongitude, latitude, and altitude, and can include factors such as thealtitude of the DTV transmitters.

The three pseudo-range measurements pr1, pr2 and pr3 are given by

 pr 1=r 1+T  (2)pr 2 =r 2 +T  (3)pr 3 =r 3 +T  (4)The three ranges can be expressed asr 1=|X−X 1|  (5)r 2 =|X−X 2|  (6)r 3 =|X−X 3|  (7)where X represents the two-dimensional vector position (x, y) of userterminal, X1 represents the two-dimensional vector position (x1, y1) ofDTV transmitter 106A, X2 represents the two-dimensional vector position(x2, y2) of DTV transmitter 106B, and X3 represents the two-dimensionalvector position (x3, y3) of DTV transmitter 106N. These relationshipsproduce three equations in which to solve for the three unknowns x, y,and T. DTV locations server 110 solves these equations according toconventional well-known methods. In an E911 application, the position ofuser terminal 102 is transmitted to E911 location server 116 fordistribution to the proper authorities. In another application, theposition is transmitted to user terminal 102.

In another implementation, user terminal 102 does not computepseudo-ranges, but rather takes measurements of the DTV signals that aresufficient to compute pseudo-range, and transmits these measurements toDTV location server 110. DTV location server 110 then computes thepseudo-ranges based on the measurements, and computes the position basedon the pseudo-ranges, as described above.

Position Location Performed by User Terminal

In another implementation, the position of user terminal 102 is computedby user terminal 102. In this implementation, all of the necessaryinformation is transmitted to user terminal 102. This information can betransmitted to user terminal by DTV location server 110, base station104, one or more DTV transmitters 106, or any combination thereof. Userterminal 102 then measures the pseudo-ranges and solves the simultaneousequations as described above. This implementation is now described.

User terminal 102 receives the time offset between the local clock ofeach DTV transmitter and a reference clock. User terminal 102 alsoreceives information describing the phase center of each DTV transmitter106 from a database 112.

User terminal 102 receives the tropospheric propagation velocitycomputed by DTV locations server 110. In another implementation, userterminal 102 receives weather information describing the airtemperature, atmospheric pressure, and humidity in the vicinity of userterminal 102 from a weather server 114 and determines troposphericpropagation velocity from the weather information using conventionaltechniques.

User terminal 102 can also receive from base station 104 informationwhich identifies the rough location of user terminal 102. For example,the information can identify a cell or cell sector within which acellular telephone is located. This information is used for ambiguityresolution, as described below.

User terminal 102 receives DTV signals from a plurality of DTVtransmitters 106 and determines a pseudo-range between the user terminal102 and each DTV transmitter 106. User terminal 102 then determines itsposition based on the pseudo-ranges and the phase centers of thetransmitters.

In any of these of the implementations, should only two DTV transmittersbe available, the position of user terminal 102 can be determined usingthe two DTV transmitters and the offset T computed during a previousposition determination. The values of T can be stored or maintainedaccording to conventional methods.

In one implementation, base station 104 determines the clock offset ofuser terminal 102. In this implementation, only two DTV transmitters arerequired for position determination. Base station 104 transmits theclock offset T to DTV location server 110, which then determines theposition of user terminal 102 from the pseudo-range computed for each ofthe DTV transmitters.

In another implementation, when only one or two DTV transmitters areavailable for position determination, GPS is used to augment theposition determination.

Receiver Architecture

FIG. 4 depicts an implementation 400 of a sampler for use in takingsamples of received DTV signals. In one implementation, sampler 400 isimplemented within user terminal 102. In another implementation, sampler400 is implemented within monitor units 108. The sampling rate should besufficiently high to obtain an accurate representation of the DTVsignal, as would be apparent to one skilled in the art.

Sampler 400 receives a DTV signal 402 at an antenna 404. A radiofrequency (RF) amp/filter 406 amplifies and filters the received DTVsignal. A local oscillator clock 416 and mixers 408I and 408Qdownconvert the signal to produce in-phase (I) and quadrature (Q)samples, respectively. The I and Q samples are respectively filtered bylow-pass filters (LPF) 410I and 410Q. An analog-to-digital converter(ADC) 412 converts the I and Q samples to digital form. The digital Iand Q samples are stored in a memory 414.

FIG. 5 depicts an implementation 500 of a noncoherent correlator for usein searching for the correlation peak of the DTV signal samples producedby sampler 400. In one implementation, correlator 500 is implementedwithin user terminal 102. In another implementation, correlator 500 isimplemented within monitor units 108.

Correlator 500 retrieves the I and Q samples of a DTV signal from memory414. Correlator 500 processes the samples at intermediate frequency(IF). Other implementations process the samples in analog or digitalform, and can operate at intermediate frequency (IF) or at baseband.

A code generator 502 generates a code sequence. In one implementation,the code sequence is a raised cosine waveform. The code sequence can beany known digital sequence in the ATSC frame. In one implementation, thecode is a synchronization code. In one implementation, thesynchronization code is a Field Synchronization Segment within an ATSCdata frame. In another implementation, the synchronization code is aSynchronization Segment within a Data Segment within an ATSC data frame.In still another implementation, the synchronization code includes boththe Field Synchronization Segment within an ATSC data frame and theSynchronization Segments within the Data Segments within an ATSC dataframe.

In other implementations, the code sequence comprises one or morepseudo-noise sequences in the DTV signal. For example, the code sequencecan include the new L5 GPS signals or similar signals, as described indetail below. In some implementations, the code sequence includes such apseudo-noise signal and one or more other signals such as the FieldSynchronization Segment within an ATSC data frame and theSynchronization Segment within a Data Segment within an ATSC data frame.

Other components of the DTV signal, such as pilot, symbol clock, orcarrier, can be used for position location. However, the use of suchsignals, which have a high repetition rate, produces inherentambiguities. Techniques for resolving such ambiguities are well-known inthe art. One such technique is disclosed in M. Rabinowitz, PhD Thesis: ADifferential Carrier Phase Navigation System Combining GPS with LowEarth Orbit Satellites for Rapid Resolution of Integer CycleAmbiguities, 2000, Department of Electrical Engineering, StanfordUniversity, pages 59-76.

Mixers 504I and 504Q respectively combine the I and Q samples with thecode generated by code generator 502. The outputs of mixers 504I and504Q are respectively filtered by filters 506I and 506Q and provided tosummer 507. The sum is provided to square law device 508. Filter 509performs an envelope detection for non-coherent correlation, accordingto conventional methods. Comparator 510 compares the correlation outputto a predetermined threshold. If the correlation output falls below thethreshold, search control 512 causes summer 514 to add additional pulsesto the clocking waveform produced by clock 516, thereby advancing thecode generator by one symbol time, and the process repeats. In apreferred embodiment, the clocking waveform has a nominal clock rate of10.76 MHz, matching the clock rate or symbol rate the received DTVsignals.

When the correlation output first exceeds the threshold, the process isdone. The time offset that produced the correlation output is used asthe pseudo-range for that DTV transmitter 106.

In receiver correlators and matched filters there are two importantsources of receiver degradation. The user terminal local oscillator isoften of relatively poor stability in frequency. This instabilityaffects two different receiver parameters. First, it causes a frequencyoffset in the receiver signal. Second, it causes the received bitpattern to slip relative to the symbol rate of the reference clock. Bothof these effects can limit the integration time of the receiver andhence the processing gain of the receiver. The integration time can beincreased by correcting the receiver reference clock. In oneimplementation a delay lock loop automatically corrects for the receiverclock.

In another implementation a NCO (numerically controlled oscillator) 518adjusts the clock frequency of the receiver to match that of theincoming received signal clock frequency and compensate for drifts andfrequency offsets of the local oscillator in user terminal 102.Increased accuracy of the clock frequency permits longer integrationtimes and better performance of the receiver correlator. The frequencycontrol input of NCO 518 can be derived from several possible sources, areceiver symbol clock rate synchronizer, tracking of the ATSC pilotcarrier, or other clock rate discriminator techniques installed in NCO518.

Position Location Enhancements

FIG. 6 illustrates a simple example of a position location calculationfor a user terminal 102 receiving DTV signals from two separate DTVantennas 106A and 106B. Circles of constant range 602A and 602B aredrawn about each of transmit antennas 106A and 106B, respectively. Theposition for a user terminal, including correction for the user terminalclock offset, is then at one of the intersections 604A and 604B of thetwo circles 602A and 602B. The ambiguity is resolved by noting that basestation 104 can determine in which sector 608 of its footprint (that is,its coverage area) 606 the user terminal is located. Of course if thereare more than two DTV transmitters in view, the ambiguity can beresolved by taking the intersection of three circles.

In one implementation, user terminal 102 can accept an input from theuser that gives a general indication of the area, such as the name ofthe nearest city. In one implementation, user terminal 102 scansavailable DTV channels to assemble a fingerprint of the location. Userterminal 102 compares this fingerprint to a stored table that matchesknown fingerprints with known locations to identify the current locationof user terminal 102.

In one implementation the position location calculation includes theeffects of ground elevation. Thus in terrain with hills and valleysrelative to the phase center of the DTV antenna 106 the circles ofconstant range are distorted. FIG. 7 depicts the effects of a singlehill 704 on a circle of constant range 702 for a DTV transmitter 106that is located at the same altitude as the surrounding land.

The computations of user position are easily made by a simple computerhaving as its database a terrain topographic map which allows thecomputations to include the effect of user altitude on the surface ofthe earth, the geoid. This calculation has the effect of distorting thecircles of constant range as shown in FIG. 7.

ATSC Signal Description

The current ATSC signal is described in “ATSC Digital TelevisionStandard and Amendment No. 1,” Mar. 16, 2000, by the Advanced TelevisionSystems Committee. The ATSC signal uses 8-ary Vestigial SidebandModulation (8VSB). The symbol rate of the ATSC signal is 10.762237 MHz,which is derived from a 27.000000 MHz clock. The structure 800 of theATSC frame is illustrated in FIG. 8. The frame 800 consists of a totalof 626 segments, each with 832 symbols, for a total of 520832 symbols.There are two field synchronization segments in each frame. Followingeach field synchronization segment are 312 data segments. Each segmentbegins with 4 symbols that are used for synchronization purposes.

The structure 900 of the field synchronization segment is illustrated inFIG. 9. The two field synchronization segments 900 in a frame 800 differonly to the extent that the middle set of 63 symbols are inverted in thesecond field synchronization segment.

The structure 1000 of the data segment is illustrated in FIG. 10. Thefirst four symbols of data segment 1000 (which are −1, 1, 1, −1) areused for segment synchronization. The other 828 symbols in data segment1000 carry data. Since the modulation scheme is 8VSB, each symbolcarries 3 bits of coded data. A rate ⅔ coding scheme is used.

Implementations of the invention can be extended to use futureenhancements to DTV signals. For example, the ATSC signal specificationallows for a high rate 16VSB signal. However, the 16VSB signal has thesame field synch pattern as the 8VSB signal. Therefore, a singleimplementation of the present invention can be designed to work equallywell with both the 8VSB and the 16VSB signal.

The 8VSB signal is constructed by filtering. The in-phase segment of thesymbol pulse has a raised-cosine characteristic, as described in J. G.Proakis, Digital Communications, McGraw-Hill, 3^(rd) edition, 1995. Thepulse can be described as $\begin{matrix}{{p(t)} = {\sin\quad{c\left( \frac{\pi\quad t}{T} \right)}\frac{\cos\left( \frac{\pi\quad\beta\quad t}{T} \right)}{1 - \frac{4\beta^{2}t^{2}}{T^{2}}}}} & (8)\end{matrix}$where T is the symbol period $\begin{matrix}{T = \frac{1}{10.76 \times 10^{6}}} & (9)\end{matrix}$and β=0.5762. This signal has a frequency characteristic $\begin{matrix}{{{P(f)} = \begin{Bmatrix}{\quad{T\quad\left( {0 \leq {f} \leq \frac{1 - \beta}{2T}} \right)}\quad} \\{\frac{T}{2}\left\{ {1 + {\cos\left\lbrack {\frac{\pi\quad T}{\beta}\left( {{f} - \frac{1 - \beta}{2T}} \right)} \right\rbrack}} \right\}\left( {\frac{1 - \beta}{2T} \leq {f} \leq \frac{1 + \beta}{2T}} \right)} \\{\quad{0\quad\left( {{f} > \frac{1 + \beta}{2T}} \right)}\quad}\end{Bmatrix}}\quad} & (10)\end{matrix}$from which it is clear that the one-sided bandwidth of the signal is(1+β)10.762237 MHz=5.38 MHz+0.31 MHz. In order to create a VSB signalfrom this in-phase pulse, the signal is filtered so that only a smallportion of the lower sideband remains. This filtering can be describedas:P _(v)(f)=P(f)(U(f)−H _(α)(f))  (11)where $\begin{matrix}{{U(f)} = \begin{Bmatrix}{1,{f \geq 0}} \\{0,{f < 0}}\end{Bmatrix}} & (12)\end{matrix}$where H_(α)(f) is a filter designed to leave a vestigal remainder of thelower sideband. A plot of the gain function for H_(α)(f) is shown inFIG. 11. The filter satisfies the characteristics H_(α)(−f)=−H_(α)(f)and H_(α)(f)=0, f>α.

The response U(f)P(f) can be represented as $\begin{matrix}{{{U(f)}{P(f)}} = {\frac{1}{2}\left( {{P(f)} + {j\quad{\overset{\Cup}{P}(f)}}} \right)}} & (13)\end{matrix}$where {hacek over (P)}(f)=−j sgn(f)P(f) is the Hilbert transform ofP(f). The VSB pulse may be represented as $\begin{matrix}{{P_{v}(f)} = {{\frac{1}{2}{X(f)}} + {\frac{j}{2}\left( {{\overset{\Cup}{X}(f)} + {2{X(f)}{H_{\alpha}(f)}}} \right)}}} & (14)\end{matrix}$and the baseband pulse signal $\begin{matrix}{{p_{v}(t)} = {{{\frac{1}{2}{x(t)}} + {\frac{j}{2}\left( {{\overset{\Cup}{x}(t)} + {x_{\alpha}(t)}} \right)}} = {{p_{vi}(t)} + {j\quad{p_{vq}(t)}}}}} & (15)\end{matrix}$where p_(vi)(t) is the in-phase component, p_(vq)(t) is the quadraturecomponent, and $\begin{matrix}{{x_{\alpha}(t)} = {2{\int_{- \alpha}^{\alpha}{{X(f)}{H_{\alpha}(f)}{\mathbb{e}}^{{j2\pi}\quad f\quad t}\quad{\mathbb{d}f}}}}} & (16)\end{matrix}$

Before the data is transmitted, the ATSC signal also embeds a carriersignal, which has −11.5 dB less power than the data signal. This carrieraids in coherent demodulation of the signal. Consequently, thetransmitted signal can be represented as: $\begin{matrix}{{s(t)} = {{\sum\limits_{n}{C_{n}\left\{ {{{p_{vi}\left( {t - {nT}} \right)}{\cos\left( {\omega\quad t} \right)}} - {{p_{vq}\left( {t - {nT}} \right)}{\sin\left( {\omega\quad t} \right)}}} \right\}}} + {A\quad{\cos\left( {\omega\quad t} \right)}}}} & (17)\end{matrix}$where C_(n) is the 8-level data signal.Monitor Units

FIG. 12 depicts an implementation 1200 of monitor unit 108. An antenna1204 receives GPS signals 1202. A GPS time transfer unit 1206 develops amaster clock signal based on the GPS signals. In order to determine theoffset of the DTV transmitter clocks, a NCO (numerically controlledoscillator) field synchronization timer 1208A develops a mastersynchronization signal based on the master clock signal. The mastersynchronization signal can include one or both of the ATSC segmentsynchronization signal and the ATSC field synchronization signal. In oneimplementation, the NCO field synchronization timers 1208A in all of themonitor units 108 are synchronized to a base date and time. Inimplementations where a single monitor unit 108 receives DTV signalsfrom all of the same DTV transmitters that user terminal 102 does, it isnot necessary to synchronize that monitor unit 108 with any othermonitor unit for the purposes of determining the position of userterminal 102. Such synchronization is also unnecessary if all of themonitor stations 108, or all of the DTV transmitters, are synchronizedto a common clock.

A DTV antenna 1212 receives a plurality of DTV signals 1210. In anotherimplementation, multiple DTV antennas are used. An amplifier 1214amplifies the DTV signals. One or more DTV tuners 1216A through 1216Neach tunes to a DTV channel in the received DTV signals to produce a DTVchannel signal. Each of a plurality of NCO field synchronization timers1208B through 1208M receives one of the DTV channel signals. Each of NCOfield synchronization timers 1208B through 1208M extracts a channelsynchronization signal from a DTV channel signal. The channelsynchronization signal can include one or more pseudonoise codes, theATSC segment synchronization signal, the ATSC field synchronizationsignal, or any combination thereof. Note that the pilot signal andsymbol clock signal within the DTV signal can be used as acquisitionaids.

Each of a plurality of summers 1218A through 1218N generates a clockoffset between the master synchronization signal and one of the channelsynchronization signals. Processor 1220 formats and sends the resultingdata to DTV location server 110. In one implementation, this dataincludes, for each DTV channel measured, the identification number ofthe DTV transmitter, the DTV channel number, the antenna phase centerfor the DTV transmitter, and the clock offset. This data can betransmitted by any of a number of methods including air link and theInternet. In one implementation, the data is broadcast in spare MPEGpackets on the DTV channel itself.

Software Receivers

One thorough approach to mitigating the effects of multipath is tosample an entire autocorrelation function, rather than to use only earlyand late samples as in a hardware setup. Multipath effects can bemitigated by selecting the earliest correlation peak.

In the case that position can be computed with a brief delay, such as inE911 applications, a simple approach is to use a software receiver,which samples a sequence of the filtered signal, and then processes thesample in firmware on a DSP.

FIG. 13 illustrates one implementation 1300 for tracking in software. Anantenna 1302 receives a DTV signal. Antenna 1302 can be a magneticdipole or any other type of antenna capable of receiving DTV signals. Abandpass filter 1304 passes the entire DTV signal spectrum to an LNA1306. In one implementation, filter 1304 is a tunable bandpass filterthat passes the spectrum for a particular DTV channel under the controlof a digital signal processor (DSP) 1314.

A low-noise amplifier (LNA) 1306 amplifies and passes the selectedsignal to a DTV channel selector 1308. DTV channel selector 1308 selectsa particular DTV channel under the control of DSP 1314, and filters anddownconverts the selected channel signal from UHF (ultra-high frequency)to IF (intermediate frequency) according to conventional methods. Anamplifier (AMP) 1310 amplifies the selected IF channel signal. Ananalog-to-digital converter and sampler (A/D) 1312 produces digitalsamples of the DTV channel signal s(t) and passes these samples to DSP1314.

Now the processing of the DTV channel signal by DSP 1314 is describedfor a coherent software receiver. A nominal offset frequency for thedownconverted sampled signal is assumed. If this signal is downconvertedto baseband, the nominal offset is 0 Hz. The process generates thecomplete autocorrelation function based on samples of a signal s(t). Theprocess may be implemented far more efficiently for a low duty factorsignal. Let T_(i) be the period of data sampled, ω_(in) be the nominaloffset of the sampled incident signal, and let ω_(offset) be the largestpossible offset frequency, due to Doppler shift and oscillator frequencydrift. The process implements the pseudocode listed below.

-   -   R_(max)=0    -   Create a complex code signal        S _(code)(t)=Σ{overscore (C)} _(n) {p _(vi)(t−nT _(i))+jp        _(vq)(t−nT _(i))}        where {overscore (C)}_(n) is zero for all symbols corresponding        to data signals and non-zero for all symbols corresponding to        synchronization signals.    -   For ω=ω_(in)−ω_(offset) to        $\omega_{in} + {\omega_{offset}\quad{step}\quad 0.5\frac{\pi}{T_{i}}}$        -   Create a complex mixing signal            s _(mix)(t)=cos(ωt)+j sin(ωt),t=[0 . . . T _(i)]        -   Combine the incident signal s(t) and the mixing signal            s_(mix)(t)            s _(comb)(t)=s(t)s _(mix)(t)        -   Compute the correlation function R(τ)=s_(code) *s _(comb)(τ)        -   If max_(r) |R(τ)|>R _(max),            R _(max)→max_(τ) |R(τ)|, R _(store)(τ)=R(τ)    -   Next ω

Upon exit from the process, R_(store)(τ) will store the correlationbetween the incident signal s(t) and the complex code signals_(code)(t). R_(store)(τ) may be further refined by searching oversmaller steps of ω. The initial step size for ω must be less then halfthe Nyquist rate $\frac{2\pi}{T_{i}}.$

The time offset τ that produces the maximum correlation output is usedas the pseudo-range.

A technique for generating the non-coherent correlation in software isnow described. This approach emulates the hardware receivers of FIGS. 4and 5. Note that while the I and Q channels are treated separately inthe block diagrams, the I and Q components may be combined to generatethe mixing signal in software. Since the non-coherent correlator usesenvelope detection, it is not necessary to search over a range ofintermediate frequencies. The process implements the pseudocode listedbelow.

Create the in-phase and quadrature code signals c_(i)(t)=Σ{overscore(C)}_(n)p_(vi)(t−nT_(i)), c_(q)(t)=Σ{overscore (C)}_(n)p_(vq)(t−nT_(i))where the sum is over n, {overscore (C)}_(n)is zero for all symbolscorresponding to data signals and non-zero for all symbols correspondingto synchronization signals. Note that c_(i) has autocorrelation R_(i),c_(q) has autocorrelation R_(q), and that their cross-correlation isR_(q).

-   -   For τ=0 to T_(per) step T_(samp) where T_(per) is the period of        the code being used, and T_(samp) is the sample interval        -   Create a reference code mixing signal            s _(mix)(t)=c _(i)(t+τ)cos(ωt+υt+φ)+c _(q)(t+τ)sin(ωt+υt+φ)        -    where ω is the nominal IF frequency of the incident signal,            υ is the frequency offset of the mixing signal relative to            the incident signal, and φ is the phase offset of the mixing            signal from the incident signal.        -   Combine the incident signal s(t) and the reference code            mixing signal s_(mix)(t).            s _(comb)(t)=s(t)s _(mix)(t)        -   Low-pass filter s_(comb)(t) to generate S_(filt)(t) such            that the expected value of S_(filt)(t) is given by            E[s_(filt)(t)]=2R_(i)(τ)cos(υt+φ))+2R_(iq)(τ)sin(υt+φ) where            we have used that fact that R_(i)(τ)=−R_(q)(τ)        -   Perform envelope detection on s_(filt)(t) (for example, by            squaring and filtering) to generate the non-coherent            correlation: z(τ)=2[R_(i)(τ)²+R_(iq)(τ)²]        -   Next τ

The time offset τ that produces the maximum correlation output is usedas the pseudo-range.

Notice that the non-coherent correlation z(τ) makes use of the signalpower in both the in-phase and quadrature components. However, as aresult of this, the effective bandwidth of the signal that generates thenon-coherent correlation is halved. The output of the non-coherentcorrelator is illustrated in FIG. 14. The upper plot shows thecorrelation peak for an interval of roughly 8×10⁻⁵ seconds. The upperplot shows the effective 3 MHz bandwidth of the correlation peak.

Experimental Results

A technique similar to that described above for tracking in software wasapplied to DTV transmissions arising from San Jose, Calif. and receivedindoors in Palo Alto, Calif. This example is presented for illustrationpurposes and not to limit the scope of the present invention. FIG. 15displays an example spectrum for a 1 millisecond sample of the signalfrom a KICU channel 52 DTV broadcast from San Jose. The signal wasdownconverted to a center frequency of 27 MHz, which corresponds to adigital frequency of 0.54 for a sampling rate of 100 mega-samples persecond. The signal was digitally bandpass filtered to a bandwidth of 6MHz.

The computed autocorrelation function for the in-phase and quadraturecomponent of the resulting 6 MHz signal is illustrated in FIG. 16. Notethat this is the autocorrelation for only the 4 data synchronizationsymbols at the beginning of each segment.

The characteristics of the 6 MHz signal are shown in FIG. 17. FIG. 17displays a portion of the autocorrelation peak for the in-phase channel.From the smoothness of the curve, one can see that the signal-to-noiseratio is high. In addition, the curvature of the peak indicates the highsignal bandwidth which makes this signal robust to multipath.

FIG. 18 depicts the results of a simulation of the operation ofcorrelator 500. The simulation was conducted using Mathematica softwareproduced by Wolfram Research. The simulation input is the digital I andQ samples stored in a memory 414 by sampler 400.

FIG. 18 shows the noncoherent correlation result for symbol-synchronoussampling at a 10.76 MHz complex sample rate and an integration time of242 milliseconds or 10 fields. The simulation is a worst case where thesamples are offset by ½ symbol or 0.05 microseconds.

The simulation also includes Gaussian noise and a signal-to-noise ratio(SNR) in the 6 MHz bandwidth of −27 dB. With a phase offset of thesampling this result degrades by 2 dB but clearly the performance wouldstill be excellent. Normal DTV reception requires a SNR of approximately+18 dB. Correlator 500 can recover tracking information at a SNR18+27=45 dB below normal DTV. This result requires accurate correctionof the sampling clock if a matched filter is employed. However, atime-gated delay lock loop (DLL) will automatically synchronize itsclock to that of the received signal and produce the same result.

Identifying the Television Transmitters

In some geographic regions DTV signals are blocked to a considerableextent by large hills and mountains. In such regions it is sometimesdesirable to augment the DTV signals broadcast from large TV towers withdigitally regenerated signals transmitted from smaller TV repeatertowers which are much closer to the blocked users. The DTV signals fromthe repeater towers are digitally regenerated and synchronized with anappropriate time offset from the DTV signal received from the main TVtower.

FIG. 19 shows the general configuration of the digital TV repeater. InFIG. 19 the main TV tower 1902 transmits the main DTV signal 1904 withits high power signal broadcast from a very large tower. Most of thenearby community is well served by all of the channels broadcast fromtower 1902. For example, house 1906 receives signal 1904 with noblockage. However in this example hilly terrain blocks the TV signalfrom main tower 1902 to house 1908 in the valley region as shown in FIG.19. A smaller TV repeater tower 1910 receives the digitized TV signalfrom main tower 1902, for example by cable 1914, regenerates themodulated signal and retransmits the TV signal as repeated DTV signal1912, which is received by house 1908 with no blockage. Thus the homesin the valley now receive a strong signal with excellent reception.

House 1906 can receive both signals: the strong repeated DTV signal 1912from nearby repeater 1910, and the weaker main DTV signal 1904 from mainTV tower 1902. A TV receiver adaptive equalizer can equalize bothsignals. This equalization is especially powerful when a new proposedpseudonoise equalization channel is utilized, as described in detailbelow.

Proposed Modification to ATSC DTV Signal

The ATSC DTV signal is the standard for DTV in the United States and anumber of other countries. In general this signal is a 6 MHz bandwidthsignal generated using 8-ary vestigial sideband (VSB) modulation witherror coding and raised-cosine spectral shaping. This signal containsembedded segment and field synchronization signals, and operates with anominal 10.76 Msps symbol rate. These signals are transmitted in thestandard VHF and UHF television bands. The signals carry MPEG-2 packetsand can carry both standard and high definition TV signals.

The DTV signal has been coded using error correction coding andinterleaving to tolerate noise. The field synchronization signal is usedboth for timing and as a training sequence for the adaptive equalizer.Equalization is essential in many areas because of significantmultipath.

A very simple modification to the ATSC transmitter has been proposedthat would not measurably impact the performance of existing digital TVreceivers nor change the current framing structure. This technique will,however, allow for the design of new types of adaptive equalizers thatcan dramatically improve the performance of digital TV receivers inrapidly changing multipath channels, and therefore greatly expand theuse of digital TV in mobile environments. This proposed ATSCpseudo-noise (PN) dithering has several objectives.

Provide a powerful new equalization signal by transmitting a low levelcontinuous known PN sequence with excellent autocorrelation properties.Because this signal is transmitted continuously, it permits equalizationeven from moving vehicles.

Provide a unique PN sequence identifier. This identifier is especiallyuseful for on-channel repeaters, but can be used for all TVtransmissions. Each TV transmitter in the US including all repeatertowers would have a unique code identifier. This permits each repeatersignal to be distinguished from the others and from the main transmittedsignal.

Negligible impact on conventional digital TV receivers. The new signalis completely backward compatible with the present ATSC signal. Nomodification to TV receivers is required. However, just as the presenceof the ghost-canceling reference GCR signal in analog TV permitsimproved performance, so also does this PN dither permit improved ATSCsignal reception in marginal areas.

Embodiments of the present invention employ the new PN codes in the DTVsignal for different reasons: to determine the position of a userterminal according to the techniques described above, and to identifythe transmitter of each DTV signal, for example to distinguish a main TVtower from a repeater tower. In addition the proposed PN ditheringtechnique can provide a low data rate 50 bps broadcast data channel tofixed and mobile users.

In a preferred embodiment the PN codes are similar or identical to oneor both of the new GPS L5 channel codes, which are described in a paperby J. Spilker, Jr, and A. J. van Dierendonck entitled “Proposed New L5Civil GPS Codes,” published in the Institute of Navigation Journal, Fall2001, Vol 48., No. 3, pages 135-143, and incorporated herein byreference. However, other PN codes can be used.

Modifying the ATSC Signal

Two techniques for adding the PN synchronization signal to the ATSCsignal are described below. In the first approach, a raised cosinesingle side-band (SSB) signal is added to the in-phase DTV signal. Thiscreates a small binary dither to the desired digital TV signal. Howeverif the PN amplitude is kept sufficiently small, for example less than{fraction (1/20)}th that of the DTV signal, the degradation is small andnot noticeable.

In the second approach, the PN sequence is added in phase quadrature tothe DTV signal. In this second approach, the PN signal can be largerthan the first approach, and causes virtually no degradation to the DTVsignal except for a small reduction in effective power (by less than 0.5dB or less).

Both techniques can be used to transmit one or both PN components of theGPS L5 signal. The data modulated signal can be transmitted in phasequadrature while the other PN code is transmitted at low level on thein-phase channel. Of course, the opposite pairing can be used instead.The first approach is now described.

In the 8-VSB modulation in the ATSC standard, three encoded bits areused to define the 8 level symbol that is used to amplitude modulate acarrier. The symbol rate is 10.76 Msps (Mega symbols per second). Aknown binary pseudo-noise sequence is selected that consists of bitscalled “chips,” where the chip rate is also 10.76 Mcps (Mega chips persecond). This binary sequence of chips is then used to create a smallbinary shift in the amplitude of the least significant bit of the 8level symbol sequence that is amplitude modulating the carrier. The neteffect is the creation of a small signal coherently added to the primaryATSC transmitter signal.

The amplitude shifts can be small enough so this additional signal is 25dB below the level of the primary ATSC signal. This should be smallenough to have little impact on reception quality of any digital TVreceiver.

FIG. 20 shows an apparatus 2000 for adding the PN synchronization signalto the ATSC signal according to the first approach. Apparatus 2000comprises a forward error correction (FEC) coder 2002, an 8-ary coder2004, a summer 2006, a pseudo-noise (PN) coder 2008, an in-phase channelvestigial sideband (VSB) coder 2010, a quadrature channel VSB coder2012, mixers 2014 and 2016, intermediate frequency (IF) oscillator 2008,summer 2020, frequency converter and high-power amplifier 2022, andtransmitter tower 2024. Apparatus 2000 is similar to a conventional ATSCmodulator and transmitter, with the exception of summer 2006 and PNcoder 2008, which simply add a low level signal to the in-phase andquadrature signal components of the conventional ATSC signal. Forexample, summer 2006 can be implemented by simply toggling theleast-significant bit in the digital stream emanating from the I and Qchannels.

Preferably the PN codes added by PN coder 2008 are the new L5 GPS codes.The L5 signal are the most powerful and precise of the GPS civilsignals. The L5 signal comprises two quasi-orthogonal codes transmittedin phase quadrature. Each code is a modified Gold code of length 10230chips, exactly 10 times as long as the present GPS C/A code. The codesare transmitted at a rate of 10.23 Mcps. In the ATSC variation, the codechip rate is changed slightly to 10.76 Mcps to match the exact symbolrate of the ATSC signal.

One of these L5 codes carries rate ½ coded data at a 50 bps data rate.The quadrature channel has no data modulation in order to permit evenlonger correlator integration times than that permitted with coded datamodulation (10 ms).

There are over 5000 quasi-orthogonal codes to select. Thus eachtransmitter or on-channel repeater can have a separate code that servesas a unique address for the transmitter.

The equalizer performance in normal TV operation is greatly improvedprovided that the training sequence is sufficiently high in amplitude.In normal operation the field synchronization signal appears as a 832bit sequence every 313 segments or every 24 ms. If the channel changesmore rapidly than that the equalizer cannot track the changes.

A new receiver can be designed to use this small pseudo-noise chipsequence to aid its channel equalizer in overcoming multipath. Acorrelator integrating this chip sequence over a single segment of 832chips will have a processing gain of 29.2 dB. This correlator outputwill have 4.2 dB sequence detection output above the primary digital TVsignal. By integrating over 10 segments (773 microseconds), thecorrelator output will have 14.2 dB advantage over the primary TVsignal.

The second approach to implementing the PN sequence is to add it inphase quadrature. In normal operation the 8VSB signal has the form$\begin{matrix}{{s\lbrack t\rbrack} = {{\sum\limits_{i}^{\quad}\quad{d_{i}\left\lbrack {{{g\left( {t - {iT}} \right)}{\cos\left\lbrack {\omega_{o}t} \right\rbrack}} + {{h\left( {t - {iT}} \right)}{\sin\left\lbrack {\omega_{o}t} \right\rbrack}}} \right\rbrack}} + {a\quad{\cos\left\lbrack {\omega_{o}t} \right\rbrack}}}} & (18)\end{matrix}$where the g term is the raised cosine waveform and h is the Hilberttransform. The d terms represent the 8VSB signal and its embeddedsynchronization signals. The last term is the pilot carrier withamplitude a.

The PN signal is added in phase quadrature with PN bits p to form thesignal $\begin{matrix}{{s\lbrack t\rbrack} = {{\sum\limits_{i}^{\quad}\quad{d_{i}\left\lbrack {{{g\left( {t - {iT}} \right)}{\cos\left\lbrack {\omega_{o}t} \right\rbrack}} - {{h\left( {t - {iT}} \right)}{\sin\left\lbrack {\omega_{o}t} \right\rbrack}}} \right\rbrack}} + {b{\sum\limits_{i}^{\quad}\quad{p_{i}\left\lbrack {{{g\left( {t - {iT}} \right)}{\cos\left\lbrack {{\omega_{o}t} + {\pi/2}} \right\rbrack}} - {{h\left( {t - {iT}} \right)}{\sin\left\lbrack {{\omega_{o}t} + {\pi/2}} \right\rbrack}}} \right\rbrack}}} + {a\quad{\cos\left\lbrack {\omega_{o}t} \right\rbrack}}}} & (19)\end{matrix}$

The phase of the PN SSB signal is rotated by 90 degrees relative to thedigital TV SSB carrier, keeping the upper sideband in both signals. Thesame phase relationship between the PN signal and its Hilbert transformis kept so as to retain the upper sideband. This equation can be writtenas $\begin{matrix}{{s\lbrack t\rbrack} = {{\sum\limits_{i}^{\quad}\quad{d_{i}\left\lbrack {{{g\left( {t - {iT}} \right)}{\cos\left\lbrack {\omega_{o}t} \right\rbrack}} - {{h\left( {t - {iT}} \right)}{\sin\left\lbrack {\omega_{o}t} \right\rbrack}}} \right\rbrack}} + {b{\sum\limits_{i}^{\quad}\quad{p_{i}\left\lbrack {{{g\left( {t - {iT}} \right)}\left( {- {\sin\left\lbrack {\omega_{o}t} \right\rbrack}} \right)} - {{h\left( {t - {iT}} \right)}{\cos\left\lbrack {\omega_{o}t} \right\rbrack}}} \right\rbrack}}} + {a\quad{\cos\left\lbrack {\omega_{o}t} \right\rbrack}}}} & (20)\end{matrix}$

FIG. 21 shows an apparatus 2100 for adding the PN synchronization signalto the ATSC signal according to the second approach. Apparatus 2100comprises a forward error correction (FEC) coder 2102, an 8-ary coder2104, summers 2106A and 2106B, a pseudo-noise (PN) and VSB coder 2108,an in-phase channel VSB coder 2110, a quadrature channel VSB coder 2112,mixers 2114 and 2116, IF oscillator 2118, summer 2120, frequencyconverter and high-power amplifier 2122, and transmitter tower 2124.Apparatus 2100 is similar to a conventional ATSC modulator andtransmitter, with the exception of summers 2106 and PNNVSB coder 2108.

Apparatus 2100 adds a PN sequence with raised cosine waveshape andsingle sideband spectrum; therefore this signal has both in-phase andquadrature components where the quadrature component is in phase withthe main data component. However, the Hilbert transform component h[t]is uncorrelated with the main component g[t]. Thus when the signal issynchronously detected the quadrature component disappears. Thequadrature component is zero only if there is no multipath. Withmultipath of course there can be many other components. The added PNsequence has amplitude b which is small compared to that of the maindata signal. The synchronous detector output r[t] is given by$\begin{matrix}{{r\lbrack t\rbrack} = {{{s\lbrack t\rbrack}{\cos\left\lbrack {\omega_{o}t} \right\rbrack}} = \left. {\left\{ {{\sum\limits_{i}^{\quad}\quad{d_{i}\left\lbrack {{{g\left( {t - {iT}} \right)}{\cos\left\lbrack {\omega_{o}t} \right\rbrack}} - {{h\left( {t - {iT}} \right)}{\sin\left\lbrack {\omega_{o}t} \right\rbrack}}} \right\rbrack}} + {b{\sum\limits_{i}^{\quad}\quad{p_{i}\left\lbrack {{{- {g\left( {t - {iT}} \right)}}{\sin\left\lbrack {\omega_{o}t} \right\rbrack}} - {{h\left( {t - {iT}} \right)}{\cos\left\lbrack {\omega_{o}t} \right\rbrack}}} \right\rbrack}}} + {a\quad{\cos\left\lbrack {\omega_{o}t} \right\rbrack}}} \right\}{\cos\left\lbrack {\omega_{o}t} \right\rbrack}} \right|_{lowpass}}} & (21)\end{matrix}$

Removing the double frequency components yields $\begin{matrix}{{r\lbrack t\rbrack} = {\frac{1}{2}\left\{ {{\sum\limits_{i}^{\quad}\quad{d_{i}\left\lbrack {g\left( {t - {iT}} \right)} \right\rbrack}} + {b{\sum\limits_{i}^{\quad}\quad{p_{i}\left\lbrack {{- {h\left( {t - {iT}} \right)}}{\cos\left\lbrack {\omega_{o}t} \right\rbrack}} \right\rbrack}}} + a} \right\}}} & (22)\end{matrix}$

After correlating with g[t−iT] the h[t−iT] term disappears. Thus the PNsequence can be added in quadrature with essentially no interferencewith the main signal. Apparatus 2100 can also add two PN sequences as isdone for the GPS L5 signal. However in this instance the second signalwill have its main component in phase with the main data signal and itsamplitude will have to be small compared to the data signal.

The only degradation caused by this PN signal addition in phasequadrature is the small amount of added power required, or alternativelythe small amount of power reduction of the digital TV signal. Forexample, using {fraction (1/10)}th of the power for the PNsynchronization signal, the digital TV signal is reduced in power by0.45 dB, a very small price to pay for a very powerful PNsynchronization signal.

Application to Improving Digital TV Equalizers

The output of the correlators for the pseudo-noise sequence will providea snapshot of the channel multipath. This data can then be used to feedthe equalizer used to combat multipath in the reception of the primaryTV signal. The time constant of the multipath should determine thecorrelation times used to adjust the equalizer parameters.

The ATSC signal data rate is 10.76 Msps, which corresponds toapproximately 0.1 microseconds symbol time. Radio waves travel about 30meters during this symbol time. Thus a change in multipath delaydistance of 30 meters will result in a symbol time change in themultipath delay.

Consider an example where a reflecting object that causes a multipathsignal is moving at 100 Km per hour. During a one millisecond timeinterval this object moves 0.0278 meters. This distance corresponds to amultipath time delay that is a small fraction of a symbol time interval.Assuming that all objects causing multipath signals do not move muchfaster than 100 Km per hour, the multipath time delay parameters shouldnot change during a one millisecond time interval.

Changes in amplitude are expected to be on the same order as that oftime delay. Thus the amplitudes of the multipath signals should notchange significantly during a one millisecond time interval.

The most rapidly changing parameter is likely to be the signal phase.The highest carrier frequency for ATSC is less than 800 MHz. For 800 MHzthe carrier wavelength is 0.375 meters. Thus during a one millisecondinterval there is no more than 0.0741 wavelengths of distance change dueto reflecting objects moving at 100 Km per hour or less. Thus during aone millisecond period there should not be more than 27 degrees of phasechange of a multipath signal relative to a phase reference defined bythe reference carrier. Of course, if there are any errors in the carrierfrequency reference then the phases can change more rapidly.

Therefore the phase changes cause most of the dynamic changes inmultipath distortions at an ATSC receiver. If an ATSC receiver is basedon a traditional design then multipath would be measured only in onephase reference coordinate. The resulting phase changes of multipathsignals in this reference coordinate would translate to rapid changes inthe amplitudes of these signals. By viewing these signals in a twodimensional coordinate space with an arbitrary phase reference, theamplitudes vary slowly compared to the more rapidly changing phases ofmultipath signals.

Based on this simple calculation, if the correlator integration time isabout one millisecond, each one millisecond the correlator outputs canprovide parameters to the TV equalizer at a fast enough rate to allowfor the rapidly changing multipath experienced in a mobile digital TVreceiver. This type of correlation aided equalizer is also much morestable and should acquire rapidly to rapidly changing channel multipath.

Alternate Embodiments

The invention can be implemented in digital electronic circuitry, or incomputer hardware, firmware, software, or in combinations thereof.Apparatus of the invention can be implemented in a computer programproduct tangibly embodied in a machine-readable storage device forexecution by a programmable processor; and method steps of the inventioncan be performed by a programmable processor executing a program ofinstructions to perform functions of the invention by operating on inputdata and generating output. The invention can be implementedadvantageously in one or more computer programs that are executable on aprogrammable system including at least one programmable processorcoupled to receive data and instructions from, and to transmit data andinstructions to, a data storage system, at least one input device, andat least one output device. Each computer program can be implemented ina high-level procedural or object-oriented programming language, or inassembly or machine language if desired; and in any case, the languagecan be a compiled or interpreted language. Suitable processors include,by way of example, both general and special purpose microprocessors.Generally, a processor will receive instructions and data from aread-only memory and/or a random access memory. Generally, a computerwill include one or more mass storage devices for storing data files;such devices include magnetic disks, such as internal hard disks andremovable disks; magneto-optical disks; and optical disks. Storagedevices suitable for tangibly embodying computer program instructionsand data include all forms of non-volatile memory, including by way ofexample semiconductor memory devices, such as EPROM, EEPROM, and flashmemory devices; magnetic disks such as internal hard disks and removabledisks; magneto-optical disks; and CD-ROM disks. Any of the foregoing canbe supplemented by, or incorporated in, ASICs (application-specificintegrated circuits).

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.

For example, while various signals and signal processing techniques arediscussed herein in analog form, digital implementations will beapparent to one skilled in the relevant art after reading thisdescription.

For example, although one method for tracking the ATSC signal using thein-phase and quadrature channels is described, it should be clear thatone can use only the in-phase channel, only the quadrature channel orany combination of the two to provide accurate tracking. Furthermore itshould be clear that there are several methods of tracking these signalsusing various forms of conventional delay lock loops and through the useof various types of matched filters.

Implementations of the present invention exploit the low duty factor ofthe DTV signal in many ways. For example, one implementation employs atime-gated delay-lock loop (DLL) such as that disclosed in J. J.Spilker, Jr., Digital Communications by Satellite, Prentice-Hall,Englewood Cliffs N.J., 1977, Chapter 18-6. Other implementations employvariations of the DLL, including coherent, noncoherent, andquasi-coherent DLLs, such as those disclosed in J. J. Spilker, Jr.,Digital Communications by Satellite, Prentice-Hall, Englewood CliffsN.J., 1977, Chapter 18 and B. Parkinson and J. Spilker, Jr., GlobalPositioning System-Theory and Applications, AIAA, Washington, D.C.,1996, Vol. 1, Chapter 17, Fundamentals of Signal Tracking Theory by J.Spilker, Jr. Other implementations employ various types of matchedfilters, such as a recirculating matched filter.

In some implementations, DTV location server 110 employs redundantsignals available at the system level, such as pseudoranges availablefrom the DTV transmitters, making additional checks to validate each DTVchannel and pseudo-range, and to identify DTV channels that areerroneous. One such technique is conventional receiver autonomousintegrity monitoring (RAIM).

Accordingly, other embodiments are within the scope of the followingclaims.

1. A user terminal comprising: a receiver adapted to receive, at theuser terminal, a broadcast digital television signal transmitted by atelevision transmitter and comprising a pseudo-noise code; and acontroller adapted to generate a pseudorange based on the pseudo-noisecode; wherein the location of the user terminal is determined based onthe pseudorange and a location of the television transmitter.
 2. Theuser terminal of claim 1, wherein the broadcast digital televisionsignal is an American Television Standards Committee (ATSC) digitaltelevision signal.
 3. The user terminal of claim 2, wherein thepseudonoise code is a Global Positioning System L5 code.
 4. The userterminal of claim 1, further comprising: a processor adapted todetermine the location of the user terminal based on the pseudorange andthe location of the identified television transmitter.
 5. The userterminal of claim 1, further comprising: a time-gated delay-lock loopadapted to track the broadcast digital television signal.
 6. A userterminal comprising: a receiver adapted to receive, at the userterminal, a broadcast digital television signal transmitted by atelevision transmitter and comprising a pseudonoise code; and acontroller adapted to generate a pseudorange based on the broadcastdigital television signal, and to identify the television transmitterbased on the pseudonoise code; wherein the location of the user terminalis determined based on the pseudorange and a location of the identifiedtelevision transmitter.
 7. The user terminal of claim 6, wherein thebroadcast digital television signal is an American Television StandardsCommittee (ATSC) digital television signal.
 8. The user terminal ofclaim 7: wherein the controller generates the pseudorange based on aknown digital sequence comprising at least one of the pseudonoise code;a Field Synchronization Segment within an ATSC data frame, and aSynchronization Segment within a Data Segment within an ATSC data frame.9. The user terminal of claim 7, wherein the pseudonoise code is aGlobal Positioning System L5 code.
 10. The user terminal of claim 6,further comprising: a processor adapted to determine the location of theuser terminal based on the pseudorange and the location of theidentified television transmitter.
 11. The user terminal of claim 6,further comprising: a time-gated delay-lock loop adapted to track thebroadcast digital television signal.
 12. A user terminal comprising:receiver means for receiving, at the user terminal, a broadcast digitaltelevision signal transmitted by a television transmitter and comprisinga pseudo-noise code; and controller means for generating a pseudorangebased on the pseudo-noise code; wherein the location of the userterminal is determined based on the pseudorange and a location of thetelevision transmitter.
 13. The user terminal of claim 12, wherein thebroadcast digital television signal is an American Television StandardsCommittee (ATSC) digital television signal.
 14. The user terminal ofclaim 13, wherein the pseudonoise code is a Global Positioning System L5code.
 15. The user terminal of claim 12, further comprising: processormeans for determining the location of the user terminal based on thepseudorange and the location of the identified television transmitter.16. A user terminal comprising: receiver means for receiving, at theuser terminal, a broadcast digital television signal transmitted by atelevision transmitter and comprising a pseudonoise code; and controllermeans for generating a pseudorange based on the broadcast digitaltelevision signal, and to identify the television transmitter based onthe pseudonoise code; wherein the location of the user terminal isdetermined based on the pseudorange and a location of the identifiedtelevision transmitter.
 17. The user terminal of claim 16, wherein thebroadcast digital television signal is an American Television StandardsCommittee (ATSC) digital television signal.
 18. The user terminal ofclaim 17: wherein the controller means generates the pseudorange basedon a known digital sequence comprising at least one of the pseudonoisecode; a Field Synchronization Segment within an ATSC data frame, and aSynchronization Segment within a Data Segment within an ATSC data frame.19. The user terminal of claim 17, wherein the pseudonoise code is aGlobal Positioning System L5 code.
 20. The user terminal of claim 16,further comprising: processor means for determining the location of theuser terminal based on the pseudorange and the location of theidentified television transmitter.
 21. A method comprising: receiving,at a user terminal, a broadcast digital television signal transmitted bya television transmitter and comprising a pseudo-noise code; andgenerating a pseudorange based on the pseudo-noise code; wherein thelocation of the user terminal is determined based on the pseudorange anda location of the television transmitter.
 22. The method of claim 21,wherein the broadcast digital television signal is an AmericanTelevision Standards Committee (ATSC) digital television signal.
 23. Themethod of claim 22, wherein the pseudonoise code is a Global PositioningSystem L5 code.
 24. The method of claim 21, further comprising:determining the location of the user terminal based on the pseudorangeand the location of the identified television transmitter.
 25. A methodcomprising: receiving, at a user terminal, a broadcast digitaltelevision signal transmitted by a television transmitter and comprisinga pseudonoise code; generating a pseudorange based on the broadcastdigital television signal; and identifying the television transmitterbased on the pseudonoise code; wherein the location of the user terminalis determined based on the pseudorange and a location of the identifiedtelevision transmitter.
 26. The method of claim 25, wherein thebroadcast digital television signal is an American Television StandardsCommittee (ATSC) digital television signal.
 27. The method of claim 26:wherein the pseudorange is generated based on a known digital sequencecomprising at least one of the pseudonoise code; a Field SynchronizationSegment within an ATSC data frame, and a Synchronization Segment withina Data Segment within an ATSC data frame.
 28. The method of claim 26,wherein the pseudonoise code is a Global Positioning System L5 code. 29.The method of claim 25, further comprising: determining the location ofthe user terminal based on the pseudorange and the location of theidentified television transmitter.
 30. Computer-readable media embodyinginstructions executable by a computer to perform a method comprising:receiving, at a user terminal, a broadcast digital television signaltransmitted by a television transmitter and comprising a pseudo-noisecode; and generating a pseudorange based on the pseudo-noise code;wherein the location of the user terminal is determined based on thepseudorange and a location of the television transmitter.
 31. The mediaof claim 30, wherein the broadcast digital television signal is anAmerican Television Standards Committee (ATSC) digital televisionsignal.
 32. The media of claim 31, wherein the pseudonoise code is aGlobal Positioning System L5 code.
 33. The media of claim 30, whereinthe method further comprises: determining the location of the userterminal based on the pseudorange and the location of the identifiedtelevision transmitter.
 34. Computer-readable media embodyinginstructions executable by a computer to perform a method comprising:receiving, at a user terminal, a broadcast digital television signaltransmitted by a television transmitter and comprising a pseudonoisecode; generating a pseudorange based on the broadcast digital televisionsignal; identifying the television transmitter based on the pseudonoisecode; wherein the location of the user terminal is determined based onthe pseudorange and a location of the identified television transmitter.35. The media of claim 34, wherein the broadcast digital televisionsignal is an American Television Standards Committee (ATSC) digitaltelevision signal.
 36. The media of claim 35: wherein the pseudorange isgenerated based on a known digital sequence comprising at least one ofthe pseudonoise code; a Field Synchronization Segment within an ATSCdata frame, and a Synchronization Segment within a Data Segment withinan ATSC data frame.
 37. The media of claim 35, wherein the pseudonoisecode is a Global Positioning System L5 code.
 38. The media of claim 34,wherein the method further comprises: determining the location of theuser terminal based on the pseudorange and the location of theidentified television transmitter.